The present invention relates to a process for producing high purity benzene by purifying crude benzene obtained by dealkylation of hydrocarbon fraction containing aromatic hydrocarbon, and more particularly to a process for producing high purity benzene by subjecting the crude benzene fraction obtained in the dealkylation step to a combination of distillation and treatment with an active material.
Various processes are available for obtaining benzene from hydrocarbon fractions containing aromatic hydrocarbons as a raw material. One of the typical process is the so-called extraction process comprising extracting the mixed aromatic hydrocarbons, particularly of benzene, toluene and xylenes by a selective organic solvent, removing the solvent therefrom and fractionating to benzene, toluene and xylenes. Another typical process is the so-called hydrodealkylation process comprising thermally or catalytically hydrodealkylating the similar raw material and increasing a benzene yield, hydrocracking other hydrocarbon than the aromatics to light hydrocarbons, thereby obtaining a liquid phase rich in benzene, and obtaining pure benzene therefrom by distillation. The present invention concerns the latter process.
The light hydrocarbons are removed from the reaction product obtained by the hydrodealkylation process by a stabilizer to obtain a crude benzene fraction. The crude benzene fraction contains aromatic hydrocarbons such as toluene, xylenes, etc. besides benzene, heavier oil, and a very small amount of olefins and thiophenes. Though the olefins and thiophenes are in a very small amount, they remain in the pure benzene even after the distillation, deteriorating the quality of benzene. Thus, it is necessary to remove the olefins and thiophenes by clay treatment.
It is well known that it is desirable to conduct the clay treatment to remove the very small amount of the olefins and thiophenes remaining in the pure benzene freed from the heavier components, thereby obtaining benzene with a higher purity (for example, PETROLEUM REFINER, 42(1963)161). In such a clay treatment, activated clay is chiefly used at a temperature of 150.degree.-230.degree. C. under a pressure of 15-30 kg/cm.sup.2. The pressure is applied to keep the benzene in a liquid phase at the treating temperature. The clay treatment has a substantial effect; reduction in the thiophenes and considerable improvements in acid wash color and bromine index of product benzene can be obtained thereby. However, it causes sometimes a development of undesired color in the product benzene. It is a serious problem for the product to meet the standard requirements as a high purity benzene. This is because some heavier oil is formed by the clay treatment, probably due to the polymerization of olefins contained in the benzene. Though the deterioration of color of the high purity benzene by the clay treatment does not always appear, it is actually indispensable to conduct redistillation after the clay treatment to assure the quality in the industrial production. In some processes the clay treatment is carried out before the rectification of benzene, because the redistillation is required after the clay treatment. That is, the crude benzene freed from the light components by a stabilizer in the dealkylation step is subjected to clay treatment, and then pure benzene is obtained by rectification (for example, HYDROCARBON PROCESSING, November (1977),132). However, the heavier oil in the crude benzene is also brought into contact with the clay, thereby promoting the deterioration of the clay and shortening the life of clay.
In the production of pure benzene according to the extraction process, no such problem appears. The crude aromatics obtained according to the extraction process are freed from the light components and the heavy components by distillation, and then resulting mixed aromatics are subjected to clay treatment. Then, rectifications of benzene, toluene and xylenes are carried out and thus the heavy components formed by clay treatment are removed by the successive fractionation as bottoms. In other words, even in the production of pure benzene by extraction process, there exist a distillation step after the clay treatment.